1. Field of the Invention
The present invention relates to cover glass and methods for producing same.
2. Background Information
Thin flat glass has been conventionally used, for example, as cover glass for protecting liquid-crystal display screens etc. of equipment such as mobile phones, PDAs, digital cameras, and FPDs and as cover glass for casings of mobile terminals. In recent years, mobile phones and PDAs have tended to become thinner and more sophisticated in functionality, and are required to have high mechanical strength. Thus, strengthened glass prepared by chemically strengthening a thin flat glass substrate is used as the cover glass.
Such strengthened glass is chemically strengthened by ion-exchange processing, for example. Ion-exchange processing generally involves the immersion of glass in a potassium salt solution (ion-exchange salt) at around 350° C. to 550° C. so as to exchange sodium ions and/or lithium ions on the glass surface with the potassium ions or the sodium ions contained in the ion-exchange salt and thereby form a compressive-stress layer on the glass surface.
Various types of glass with different compositions have been developed as glass raw materials for producing strengthened glass. Particularly, glass having high mechanical strength and heat resistance has been proposed for the production of the aforementioned chemically-strengthened cover glass (see Patent Literature 1: JP-A No. 2010-180076, for example).
This type of strengthened glass uses ion-exchange components in combination and, for example, defines the content of Li2O+Na2O+K2O, but contains substantially no Li2O. In this way, the strain point etc. can be increased while giving consideration to characteristics such as the devitrification of glass, the coefficient of thermal expansion, the thermal shock resistance, and the stress relaxation value.
The glass transition temperature (Tg) tends to increase with the increase in strain point, which will provide the glass with favorable characteristics in terms of improvement in heat resistance. This, however, will increase the optimal ion-exchange processing temperature at which a predetermined compressive-stress value suitable for the glass composition can be achieved reliably. For example, as described in Paragraph 84 etc. of Patent Literature 1, if ion-exchange processing is carried out at relatively low temperatures (440 to 450° C.), then it will require around 6 hours to form a compressive-stress layer having a preferable depth of 20 μm or greater. This is disadvantageous in that production efficiency is significantly reduced.
By including Li2O, the glass transition point can be lowered effectively, and it is also anticipated that the ion-exchange time can be shortened. However, large amounts of Li2O may cause other problems; for example, the Li concentration in the molten salt (ion-exchange salt) at the time of ion-exchange processing may become too high, which may inhibit ion exchange and reduce compressive stress.
Meanwhile, including fairly large amounts of Li2O in the glass composition can efficiently lower the glass transition temperature (Tg) and improve the ion exchangeability. Accordingly, various kinds of glass having glass compositions containing Li2O have been proposed (see, for example, Patent Literature 2: JP-A No. 2004-131314 and Patent Literature 3: US2009-0142568A1).
As described above, a compressive-stress layer that provides strengthened glass with the necessary stress is generally formed by exchanging the sodium ions and/or lithium ions on the glass surface with potassium ions and/or sodium ions in the ion-exchange salt. Indices indicating the aforementioned stress, such as the compressive-stress value and the surface stress depth of layer, are generally found by observing the fringe pattern, which represent the glass surface's refractive index that has changed as a result of the ion-exchange processing (i.e., the birefringent characteristic), while retaining the form of the strengthened glass.
However, if the amount of Li2O is too large, then the fringe pattern caused by the refractive index cannot be observed, and thus the compressive-stress value and the surface stress depth of layer cannot be measured while retaining the form of the strengthened glass. Even if a faint fringe pattern could be observed, that would not allow the correct measurement of the compressive-stress value and the surface stress depth of layer. So, in order to measure the strength of the obtained strengthened glass, it becomes necessary to destruct the strengthened glass plate. Also, other problems emerge; for example, the steps required until the measurement can be conducted become complicated.
Accordingly, it is necessary to balance the relationship among the lowering of the ion-exchange processing temperature, the reduction of ion-exchange time, the ease of measuring the compressive-stress layer, and heat resistance, and find a trade-off.